The present invention relates to a telecommunication system comprising a main station and a plurality of substations, in which system communication between the main station and the substations occurs at a predetermined bit rate via a transmission channel on the basis of a multiple access protocol. The channel is at least partially common to the substations. For ranging the substations, the system comprises transmission means for transmitting ranging information to the main station and adjustment means for at least adjusting the timing for transmitting information to the main station on the basis of ranging control information received from the main station. The main station comprises receiving means for receiving the ranging information, sampling means for taking samples from the ranging information, and processing means for determining the ranging control information from the samples. Depending on the transmission channel used, i.e. a glass fibre, a coaxial cable or a radio link, the telecommunication system can be a passive optical network (PON), a local area network, a satellite system or a cellular mobile radio system or the like.
The present invention further relates to a main station for use in such a system.
The present invention further relates to a substation for use in such a system.
A telecommunication system of this kind is known from the International Application No. WO 91/08623, in which a TPON (Telephony on a Passive Optical Network) is disclosed comprising a head-end station broadcasting TDMA (Time Division Multiple Access) frames to a number of termination units on an optical network in a downstream direction. In an upstream direction, each termination unit transmits data in a predetermined time slot and the data from the different terminations are assembled at the head-end into a TDMA frame of a predetermined format. For compensation at start-up or in operation of the TPON of differing delays and attenuation associated with the different distances of the various terminations from the head-end station, in the TPON each termination is arranged to transmit coarse and fine ranging pulses timed to arrive in a respective predetermined portion of the upstream TDMA frame. In FIG. 2 of WO 91/08623 a so-called phase 1 ranging window for coarse ranging and a so-called phase.2 window for fine ranging within the upstream TDMA frame is shown. The head-end station is arranged to monitor the timing, i.e. phase and amplitude of the arrival of ranging pulses from each of the terminations and to return a servo-control signal to the termination to retard or advance its transmissions as appropriate and to adjust the transmission power. The frames in this TPON have a predetermined bit rate. For achieving active fine ranging in the known TPON respective received ranging pulses from the terminations are sampled in the head-end station at a high sampling rate greater than the said predetermined bit rate, and the samples are processed to determine the respective phases of the ranging pulses relative to respective expected values. The head-end generates the ranging control signals from the determined phase differences between the received and expected phases and transmits the respective control signal to the terminations, which modify the phase of transmissions from the terminations accordingly so as to avoid overlapping of data within a frame in the upstream direction. The TPON operates at 20 Mbit/s. To achieve a required fine ranging accuracy of about 0.1 bit period of a predetermined bit rate in the TPON, at least four times oversampling is required, the necessary extra accuracy being achieved by means of "dithering", i.e. averaging phase inaccuracies in the sampling and clock moments. Due to the applied oversampling Analog-to-Digital Converters, A/D-converters have to be used suitable for such high sampling rates, such A/D-converters being expensive, consuming much power and having limited resolution. For systems with even higher bit-rates, e.g. 40 Mbit/s or over, the disclosed oversampling technique for the purpose of fine ranging becomes cumbersome in achieving the required accuracies.
In the European patent application No. 0 383 557 a TPON is disclosed in which data is communicated between a central exchange and a number of remote subscriber stations in which system, typically operating at 20 Mbit/s, a single clock is used throughout the system. To achieve a 0.1 bit system accuracy the central exchange includes, on its receive side, a phase alignment system which enables the exchange to handle received data using only its local master clock despite arbitrary variations in phase. The phase alignment system comprises a lumped constant delay line formed from discrete reactive components, the delay line having taps at 0.1 bit of the system bit rate. For phase alignment purposes each tap is monitored by an eye position detection circuit. In this system data are transmitted using an extended version of the standard ATM (Asynchronous Transfer Mode), data cells in the upstream direction comprising a three bit lead-in header providing the "eye", the position of which is determined by the eye position detection circuit in order to identify the appropriate tap. Although accurate, this system, which effectively implies real time oversampling at ten times the master clock, is complicated, has similar disadvantages as to power consumption as the said WO 91/08623, and becomes cumbersome at still higher frequencies.
It is an object of the present invention to provide simple and inexpensive fine and/or coarse ranging in a telecommunication system of the above kind having the required accuracy while at the same time being suitable for future higher system master clock frequencies.